MCM-41 supported PdNi catalysts for dry reforming of methane

A series of bimetallic PdNi catalysts supported on mesoporous MCM-41 with different Ni content (Ni/Si ratio of 0.2–0.4) was synthesized. The effect of Pd addition to Ni-containing catalysts as well as the effect of the Ni content on the surface and catalytic properties of the catalysts was studied. The samples were characterized using various techniques, such as energy-dispersive X-ray spectroscopy, N₂ adsorption–desorption isotherms, X-ray diffraction, thermogravimetric and differential analyses, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and temperature-programmed reduction. Reforming of methane with carbon dioxide was used as a test reaction. The results indicated that the addition of a small amount of Pd (0.5%) to Ni-containing catalysts leads to formation of small nano-sized, easy reducible NiO particles. Agglomeration of NiO as well as of metallic nickel phase over PdNi samples increased with increasing the Ni content. Formation of filamentous carbon over surface of spent monometallic Ni and bimetallic PdNi catalyst was observed. In spite of filamentous carbon deposition, the catalytic activity and stability of bimetallic PdNi catalysts are higher than those of monometallic Ni one. Within bimetallic system, the PdNi catalyst with Ni/Si ratio of 0.3 revealed the best performance and stability caused by presence of small nickel particles well dispersed on the catalyst surface.     

Voltammetric behavior of Pd(II) and Ni(II) ions and electrodeposition of PdNi bimetal in N-butyl-N-methylpyrrolidinium dicyanamide ionic liquid

In this study, the voltammetric behavior of Pd(II), Ni(II), and mixtures of Pd(II) and Ni(II) was carried out in room-temperature N-butyl-N-methylpyrrolidinium dicyanamide ionic liquid (BMP-DCA IL). Electrodeposition of PdNi bimetal was achieved by controlled-potential electrolysis at iron wire electrodes from BMP-DCA containing various molar ratios of PdCl₂/NiCl₂. BMP-DCA shows good solubilities to PdCl₂ and NiCl₂, respectively, leading to the convenience for preparing the electrodepositing baths. By tuning the molar ratios of Pd(II)/Ni(II) in the electrodepositing baths and/or the applied potentials, PdNi coatings with various atomic contents of Pd could be obtained. Among these PdNi bimetallic coatings, the PdNi coatings with atomic ratios of ∼80/20 had the highest oxidation current in methanol oxidation reaction (MOR) in 1 M NaOH and exhibited the best poisoning tolerance. Powder X-ray diffraction analysis indicated that two separate metallic phases belonging to Pd and Ni existed in the PdNi coatings. The diffraction signal of Pd was very broad, indicating the tiny crystal size of Pd in the bimetal coatings. The scanning electron spectroscopic micrographs of PdNi coatings demonstrated that the Pd_∼80Ni_∼20 coatings had three-dimensional structures. This morphological characteristic implied that the composition and the surface morphology of the PdNi coatings equivalently contributed to the electrocatalytic activity toward MOR. The Pd_∼80Ni_∼20-coated iron-electrode (Fe/Pd_∼80Ni_∼20) was used to detect methanol and the linearity was observed in 3.53 μM to 758.88 μM using hydrodynamic chronoamperometry where a potential of −0.1 V (vs. Ag/AgCl) was applied.     

Electroless plated SnO2–Pd–Au composite thin film for room temperature H2 detection

Extremely thin SnO₂ nanosheets with high surface area were fabricated through a one-pot hydrothermal method. In this work, gas sensing property of the SnO₂ nanosheets was studied. SnO₂–Pd–Au mixed thin films were prepared by electroless deposition of Pd, Au, and nanostructured SnO₂ onto the surface of a high resistance alumina substrate. The whole fabrication process was carried out at room temperature without any thermal treatment required. The films deposited on the alumina substrate were characterized by SEM and EDS. The co-deposited Au improved the electric conductance of the sensing film. A relatively large amount of Pd (Pd/Sn ratio around 1:1) was obtained for the film instead of the usually low doping value of Pd (∼0.1% level) for SnO₂ hydrogen sensor. It has been found that the SnO₂–Pd–Au composite film sensor has fast response in the range of 134–1469 ppm toward hydrogen gas at room temperature. The sensor also shows good stability and repeatability. Effects of annealing condition of the sensing film on H₂ gas sensing performance was investigated as well. A possible machnism for SnO₂–Pd room temperature hydrogen sensing is proposed.     

Influence of temperature on hydrogen electrosorption into palladium-noble metal alloys. Part 2—Palladium–platinum alloys

Hydrogen electrosorption into Pd-rich (>75 at.% Pd in the bulk) Pd–Pt alloys obtained by electrodeposition was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. The influence of temperature (in the range between 283 and 328 K) on the amount of absorbed hydrogen, the potential of the α–β phase transition, the extent of absorption–desorption hysteresis and the potential of absorbed hydrogen oxidation was examined. It has been found that for the temperature range studied the potentials of the α → β and β → α phase transitions are shifted negatively with both increasing temperature and decreasing Pd content in the alloy bulk. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of the β-phase formation and decomposition were determined. With decreasing Pd bulk content the process of the β-phase formation becomes less exothermic and the thermodynamic stability of the β-phase decreases. The maximum hydrogen absorption capacity of Pd–Pt alloys decreases with increasing temperature and decreasing Pd bulk content. The potential of absorbed hydrogen oxidation is shifted negatively with increasing temperature and decreasing Pd bulk content.     

Synthesis of bimetallic PdAu nanoparticles for formic acid oxidation

In this work, a simple co-deposition strategy for the synthesis of carbon-supported Pd–Au alloy was reported. Our approach involves the co-reduction of Au and Pd ions using ethylene glycol and sodium citrate as the reducing and stabilizing reagents. Both alloy and non-alloy bimetallic Pd–Au nanoparticles are produced using a right rate-limiting strategy. For example, when ethylene glycol and sodium citrate are the limiting reagent with Au and Pd ions in excess, the synthesis environment favors preferential nucleation and growth of Au nanoparticles followed by deposition of Pd either as the shell of Au core or as separate Pd clusters. On the other hand, if the supply of metal ions (not the reducing reagents) limits the reaction, it creates a synthesis condition for Pd–Au alloy particles. The as-prepared Pd–Au alloys exhibit higher Pd-specific activities towards formic acid oxidation compared with the non-alloy counterpart or individual Pd catalyst and an easier removal of adsorbed oxygen species (e.g., O_ads or OH_ads) was observed from the surface of Pd–Au alloy with a higher content of Au.     

Polypropylene as a carbon source for the synthesis of multi-walled carbon nanotubes via catalytic combustion

Multi-walled carbon nanotubes (MWCNTs) were efficiently synthesized by catalytic combustion of polypropylene (PP) using nickel compounds (such as Ni₂O₃, NiO, Ni(OH)₂ and NiCO₃ · 2Ni(OH)₂) as catalysts in the presence of organic-modified montmorillonite (OMMT) at 630-830 °C. Morphologies of the sample undergoing different combustion times were observed to investigate actual process producing MWCNTs by this method. The obtained MWCNTs were characterized by X-ray diffraction (XRD), transmission electron microscope and Raman spectroscopy. The yield of MWCNTs was affected by the composition of PP mixtures with OMMT and nickel compounds and the combustion temperature. The proton acidic sites from the degraded OMMT layers due to the Hoffman reaction of the modifiers at high temperature played an important role in the catalytic degradation of PP to supply carbon sources that are easy to be catalyzed by nickel catalyst for the growth of MWCNTs. The XRD measurements demonstrated that the nickel compounds were in situ reduced into the Ni(0) state with the aid of hydrogen gas and/or hydrocarbons in the degradation products of PP, and the Ni(0) was really the active site for the growth of MWCNTs. The combination of nickel compounds with OMMT was a key factor to efficiently synthesize MWCNTs via catalytic combustion of PP.     

Pd-based (Ce,Zr)Ox-supported catalysts: Promoting effect of base metals (Cr, Cu, Ni) in CO and NO elimination

A series of Pd–M bimetallic three-way catalysts (M = Cr, Cu and Ni) supported on a (Ce,Zr)O_x material has been characterized using a combination of X-ray diffraction and Raman spectroscopy, and employing in situ diffuse reflectance Fourier transform infrared and X-ray near-edge structure spectroscopies to analyse the redox and chemical processes taking place during light-off conditions under CO, NO and O₂. The catalytic behaviour of these bimetallic systems was strongly affected by the degree of interaction between the noble and base metals in the calcined state. Among the base metals tested, Ni appeared to exert the least influence over the noble metal state/behaviour after calcination and under reaction conditions. Cr and Cu appear to interact with Pd in the calcined state, leading to a reduction in the temperature at which Pd was converted to Pd(0) with simultaneous formation of a binary PdM alloy during the reaction run. At high temperature, these alloy phases evolved into pure metallic Pd(0) particles and, in the case of the Cu-containing catalyst, result in a strong interaction with the support. The catalytic performance of these three Pd–M systems in the CO and NO elimination reactions are correlated with the nature and properties of the oxidized and reduced Pd-containing phases which are present in each case.     

Difference in the reactivity of acetaldehyde intermediates in the dehydrogenation of ethanol over supported Pd catalysts

Catalytic performances of supported Pd catalysts for the dehydrogenation of ethanol were greatly modified upon the formation of Pd alloy phases. Over Pd–Zn, Pd–Ga and Pd–In alloys, acetaldehyde was selectively produced at lower conversion levels. With the increased conversion level, ethyl acetate was produced at the expense of acetaldehyde. The selectivities for the ethyl acetate formation exceeded that over a Cu/ZnO catalyst. Over metallic Pd, the decomposition of ethanol, C₂H₅OH → CO + CH₄ + H₂, occurred to a considerable extent. It was shown that the reactivity of acetaldehyde species over the Pd alloys was markedly different from that over metallic Pd. Over the Pd alloys, acetaldehyde species were stabilized and transformed into ethyl acetate by the nucleophilic addition of ethanol. By contrast, over metallic Pd, aldehyde species were rapidly decarbonylated to methane and carbon monoxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Pd–Fe/α-Al2O3/cordierite monolithic catalyst for CO coupling to oxalate

A novel Pd–Fe/α-Al₂O₃/cordierite monolithic catalyst was prepared for the synthesis of diethyl oxalate from CO and ethyl nitrite. The palladium-based monolithic catalyst with an optimal thickness (15 μm) of Al₂O₃ washcoat showed excellent catalytic activity and selectivity in a continuous flow, fixed-bed microreactor. The physicochemical properties of catalyst were studied by a variety of characterization techniques. Catalytic performances of Pd–Fe/α-Al₂O₃/cordierite monolithic catalysts were dependent on particle size of alumina-sol, thickness of Al₂O₃ washcoat, pore structure, surface acidity of carrier, and distribution of active metal component on the Al₂O₃ washcoat. Under the mild reaction conditions, CO conversion was 32% and the space–time yield of diethyl oxalate was 429 g/(L h). Pd efficiency (DEO(g)/Pd(g)/h) of the monolithic catalyst (274 h⁻¹) was much higher than that of a reference pellet catalyst (46 h⁻¹), probably due to high dispersion of the Pd nanoparticles on the surface of the monolithic catalyst.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

Pd–Ru/C as the electrocatalyst for hydrogen peroxide reduction

Pd–Ru, Pd and Ru nanoparticles supported on Vulcan XC-72 carbon were prepared by chemical reduction of PdCl₂ and/or RuCl₃ in aqueous solution using NaBH₄ as the reducing agent. Transmission electron microscopy measurements showed that Pd–Ru particles were uniformly dispersed on carbon. The particle size of Pd–Ru is around 5–9 nm. X-ray diffraction analysis indicated that Ru formed alloy with Pd in Pd–Ru/C catalyst. The electroreduction of hydrogen peroxide on Pd–Ru/C, Pd/C and Ru/C in H₂SO₄ solution was examined by linear sweep voltammetry and chronoamperometry measurements. Results revealed that Pd–Ru/C catalyst exhibited higher electrocatalytic activity for hydrogen peroxide reduction than Pd/C and Ru/C. All the catalysts showed good stability for hydrogen peroxide electroreduction in H₂SO₄ electrolyte.     

Highly active carbon-supported PdNi catalyst for formic acid electrooxidation

A new carbon-supported PdNi (PdNi/C) catalyst is prepared by a simple simultaneous reduction reaction with sodium borohydride in glycol solution. The results show that the performance of PdNi/C catalyst for formic acid oxidation is greatly improved compared with that of Pd/C. X-ray diffraction (XRD) results show that Ni exists in the catalyst both as NiO and as PdNi alloy. The value of the apparent activation energy shows that the activity of formic acid oxidation on the PdNi/C is more sensitive to temperature compared with Pd/C.     

Preparation of alumina/carbon nanotubes composites by chemical precipitation

Continuous alumina coating on multi-walled carbon nanotubes (MWCNTs) was successfully prepared by a new method of chemical precipitation using aluminum nitrate and ammonia as starting materials. Structure and morphology of the alumina/multi-walled carbon nanotubes (Al₂O₃/MWCNTs) composites were characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), infrared spectra (IR), thermo gravimetric analysis (TG), differential thermal analysis (DTA) and N₂ adsorption–desorption. The results show that polyvinyl alcohol (PVA) modification on the surface of MWCNTs contributes to form continuous alumina coating, γ-Al₂O₃ layers with thickness of 1–3 nm cover the surface of MWCNTs and the original structure of MWCNTs is retained during the coating process.     

Growth of Pd particles in methanol synthesis over Pd/CeO2

The metal–support contact structure of Pd–CeO₂ changed with increasing the temperature of reduction. Upon high temperature reduction, severe sintering of Pd particles occurred, while sintering of the ceria support was marginal. The catalytic deactivation of the Pd/CeO₂ catalysts during methanol synthesis was caused by the further structural change in the Pd–CeO₂ contact under reaction conditions. Considerably large Pd particles (about 100 nm) were formed in the catalysts subjected to methanol synthesis, and there was a close correlation between the activity loss and the growth of the Pd particles. It was proposed that the structure of Pd–ceria contact shifted from small Pd clusters supported on ceria to sintered large Pd particles dispersed in a mass of ceria.     

Tailoring electronic properties and kinetics behaviors of Pd/N-CNTs catalysts for selective hydrogenation of acetylene

Pd catalyzed selective hydrogenation of acetylene shows remarkable electronic effects. In this work, a strategy is proposed to tailor the electronic properties of Pd nanoparticles by nitrogen doping of carbon nanotubes (CNT) support toward the improved reaction kinetics. While excluding the Pd size effects, the intrinsic promotional effects of the nitrogen doping are demonstrated, which are mainly due to the increased Pd electron density resultant from the presence of more graphitic nitrogen species based on X-ray photoelectron spectroscopy measurements and density functional theory (DFT) calculations. Kinetics analysis and C₂H₂/C₂H₄-temperature-programmed desorption (TPD) measurements reveal that the electron-rich Pd catalyst with the moderately weakened adsorption strength can give rise to the decreased activation energy and thus the simultaneously enhanced activity, selectivity, and stability. The aspects demonstrated here could guide the rational design and optimization of Pd catalysts for the selective hydrogenation of acetylene.     

Low-temperature catalytic combustion of methane over Pd/CeO2 prepared by deposition–precipitation method

Ceria supported 2 wt% Pd catalysts for low-temperature methane combustion were prepared by the impregnation (IM) and deposition–precipitation (DP) methods, which are denoted as Pd–IM and Pd–DP, respectively. DP was found to be an available method for achieving high activity and stability of the Pd/CeO₂ catalyst. The temperatures for methane ignition (T_10%) and total conversion (T_100%) over Pd–DP are 224 and 300 °C at GHSV of 50,000 h⁻¹, which are 83 and 110 °C lower than the corresponding temperatures of Pd/Al₂O₃. X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses show that palladium species in Pd–DP is highly dispersed, positively charged and difficultly reduced. Raman spectra disclosed that the largest concentration of defects and/or oxygen vacancies was formed in Pd–DP catalyst. A kind of cationic PdO^δ+ sites with higher binding energies than PdO are in close vicinity to the oxygen vacancies in the CeO₂ support and might act as the active centers for methane oxidation. Furthermore, the deactivation and steam aging tests for Pd–DP showed that the performance of this type of palladium was very stable and could be repeatedly recovered after several long time aging tests.     

Using RTILs of EMIBF4 as “water” to prepare palladium nanoparticles onto MWCNTs by pyrolysis of PdCl2

For the first time, palladium nanoparticles supported on MWCNTs (multi-walled carbon nanotubes), denoted as Pd/MWCNTs, were prepared by a simple pyrolysis process of PdCl₂ dissolved in room temperature ionic liquids (RTILs) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) rather than water. X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were used to characterize the structure of Pd/MWCNTs, and the results showed that Pd nanoparticles with highly crystalline structure and a diameter of around 4 nm were prepared, and more importantly, except for carbon and palladium no other elements were detected. The results obtained from a pyrolysis process only containing PdCl₂ and EMIBF4 testified that in our developed pyrolysis process, EMIBF4 was used not only as ligands, to form a novel complex, but also as a reducing agent, to reduce Pd²⁺. The electrocatalytic performance of Pd/MWCNTs-modified glassy carbon electrode towards ethanol oxidation reaction (EOR) was also probed by cycle voltammetry (CV), demonstrating that it was possible to utilize the obtained Pd/MWCNTs as anode materials in fuel cell. Initiating the application of RTILs in the pyrolysis process and finding that EMIBF4 could be employed as ligands and reducing agents are the main contributions of this preliminary work.     

Structural,Thermal and Magnetic Properties of Thin Metal Films and Adsorbate–Substrate Systems Studied by XAFS and XMCD

Thin metal films often exhibit interesting properties that are essentially different from the bulk ones. XAFS (X-ray absorption fine structure) and XMCD (X-ray magnetic circular dichroism) techniques are quite suitable to investigate structural, thermal and magnetic properties of thin metal films. In this proceeding, we will present following two topics concerning structural and magnetic properties of adsorbates on thin metal films. The first one is the adsorption geometry of SO₂ on a 1-monolayer (ML) Pd thin film grown on a Ni(111) single crystal. It was found by S K-edge XAFS that SO₂ is lying flat on 1-ML Pd/Ni(111). This result is not similar to the bulk Pd surface but to the bulk Ni one. This finding indicates significant modification of the electronic structure of the 1-ML Pd film compared to the bulk one. The second topic is the magnetic moment induced on CO adsorbed on Ni epitaxial films grown on Cu(001). The O K-edge XMCD results revealed that in the perpendicularly magnetized 10-ML Ni film the orbital moment of CO is parallel to the substrate Ni magnetization, while it is antiparallel in the in-planar magnetized 6-ML and thick (>100 ML) films. The origin of the induced orbital moment at CO is discussed.     

Highly dispersed heterogeneous palladium catalysts by the introduction of plant tannin into porous Al2O3 supports

A new strategy was provided by the introduction of plant tannins in porous Al₂O₃ to solve the problems of thermal migration of Pd during catalyst preparation, which ensured the preparation of heterogeneous Pd catalysts with well dispersion and superior activity. Compared with the conventional Pd–Al₂O₃ catalyst, the as-prepared heterogeneous Pd catalyst exhibited considerably improved Pd dispersion, which was highly active for the catalytic hydrogenation of olefins.     

In situ synthesis and hydrogen storage properties of PdNi alloy nanoparticles in an ordered mesoporous carbon template

Organized mesoporous carbon has been used as a nanoreactor to prepare PdNi metallic particles using an incipient wetness method starting from Pd and Ni salts. The final composite material consists of nanosized metallic particles of an alloy with composition Pd_0.60Ni_0.40 highly dispersed within the carbon host structure. The thermodynamic hydrogenation properties of both the PdNi-free OMC and the Pd_0.60Ni_0.40-OMC composite have been determined by hydrogen isotherm sorption measurements. The introduction of the palladium–nickel alloy into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 2 MPa, since the hydrogen uptake is mainly attributed to physisorption on the carbon surface. However, at room temperature and moderate pressure (0.5 MPa), the filling of the OMC with nanocrystalline Pd_0.60Ni_0.40 results in larger hydrogen uptake than that of the PdNi-free OMC.